To improve accuracy of positioning a head to a target position on the disc surface of a rotating disc, it is necessary to suppress non-repeatable run-outs (NRRO) such as disc flutters. The non-repeatable run-outs is disturbances erupting at a frequency that is asynchronous with rotation of the disc (hereinafter, referred to as suppression target frequency). As a method to suppress the non-repeatable run-outs, there is a method to add a filter for removing the non-repeatable run-outs (hereinafter, referred to as NRRO suppression filter) to a feedback system.
However, according to the method to add the NRRO suppression filter to the feedback system, the gain and phase of a sensitivity function at frequencies around the suppression target frequency do not coincide with the gain and phase of a sensitivity function without the NRRO suppression filter, whereby the accuracy of positioning the head may not be improved.